"Basically there are two ways to end up with a hoof crack, though there are really more than two," he says.
"One is to have an injury to the tissue that is responsible for creating the horn. For instance, I've got an old wound on
my left thumb and therefore a permanent defect in the thumbnail because the germinal tissue that produces horn is absent,"
Moyer explains. "You'll see horses with the same thing."
Previous injury may produce a defect, which in turn produces a permanent crack, sometimes because of an infection around the
coronary tissue, or the horse stepping on it. "Any situation that can cause trauma to your fingernail or toenail could happen
to a horse," Moyer says.
The second most common situation, Moyer explains, is that hoof cracks repre-
sent a failure of the hoof wall, where sheer forces are such that the hoof just splits. "For the most part, these are horses
with reasonably weak feet, that tend to have a low heel, longish kind of toe, flat foot and a thin hoof. That describes an
awful lot of Thoroughbreds," says Moyer. "In most cases, it's a mechanically induced injury."
Cause is critical to treatment
"Basically the repair process is two-fold," notes Moyer. "One is to try to figure out, if you can, how did it get there? The
second objective is to stabilize the crack, because the thing that hurts is motion at the crack site, initiating inflammation
and pinching below it."
O'Grady agrees that determining cause is crucial to the proper treatment. "There are as many ways to repair a quarter crack
as there are quarter cracks, but ... (learning the) cause is most important," he says.
Quarter cracks originate from three areas: limb conformation, foot conformation and landing pattern.
» LIMB CONFORMATION: It may start from the shoulder or knee down, when the limb is rotated outward in such a way that the horse lands on one side
of the foot and then puts pressure on the inside.
» FOOT CONFORMATION: A horse's foot may be offset to one side, so it bears more load on that side; or it may have a low or under-run heel, where
sufficient structure in the back of the foot is lacking. "This type of heel may be weak due to insufficient hoof-wall growth
and there may be insufficient solar surface to adequately support the palmar/plantar part of the foot," O'Grady says.
In addition there may be increased pressure in the quarter and heel during the stance phase, as a result of delayed breakover
caused by the long toe. Upright hoof conformation with high heels promotes heel-first landing, which increases pressure through
the heels and quarters.
» LANDING PATTERN: A horse with a quarter crack generally is one that lands or impacts the ground asymmetrically, impacting first on one side
of the hoof and then loading the opposite side. This strike pattern often is related to conformation, but also can be caused
by improper trimming.
Asymmetric landing may place extra forces on one side of the hoof wall, producing enough force to proximally displace the
heel bulb. Along with abnormal conformation, this landing pattern can lead to a shearing force or increased pressure within
The landing pattern and the conformation of the foot also may perpetuate quarter cracks caused by trauma to the coronary band
or a previous abscess.